In many rocket motor applications it is necessary to obtain the efficiencies associated with the optimum nozzle expansion ratio but the extra length of the nozzle exit cone that is required to reach this optimum expansion ratio cannot be tolerated in a specific missile envelope. In these cases it has been known to utilize extendible exit cones of various types. One type which has been proposed is the telescoping or "Boy Scout cup" extendible exit cone such as is shown in U.S. Pat. No. 3,561,679, Lager, Feb. 9, 1971 for Collapsible Nozzle for Aircraft Rocket Motors. In this approach, two or more exit cone segments are telescoped over each other in the stowed position and then extended rearwardly to the fully deployed position and locked in place by suitable locking fingers, all is shown in the aforementioned U.S. Patent. When an extendible exit cone such as that shown in the Lager patent was tested it was determined to be unsatisfactory for several reasons; the most important of which was that the blow-off forces generated within the extendible exit cone tended to force the lips of the locking fingers on the aft nozzle exit cone section out of engagement with the locking shoulder on the forward exit cone segment. This produced leakage of high temperature gases around the surfaces causing damage thereto and also tended to permit the exit cone to collapse upon the action of the blow-off forces. According to this invention I have provided a positive locking means for maintaining the extendible exit cone segments in their extended position which is not susceptible to unlatching under the action of blow-off forces nor is it susceptible to gas leakage.